The present invention relates to a method for producing metallic powders in micron and submicron size. The metallic powders are particularly useful for electronic applications.
Metallic powders in micron size range have many special applications including dental materials, medical prosthetics, catalysts, electronic devices such as multilayer capacitors, integrated circuits and structural applications in the aircraft industry. Purity, controlled particle size and morphology are often essential for many applications. Smooth spherical particles of palladium (Pd), silver (Ag) and palladium/silver (Pd/Ag) alloys, nickel (Ni) and gold (Au) in 0.5 to 2.0 micrometer size range are needed for multilayer capacitors. For this use, it is essential to limit the onset of oxidation to a temperature above 400.degree.C. and at the same time the total oxidation must also be below a few percent. The presence of too many small particles (0.3 microns and smaller) is undesirable because of their high surface to volume ratio which makes them more susceptible to higher levels of oxidation. Also, particles larger than 2 microns are to be avoided so that a thin smooth layer may be printed by screen printing employing pastes made with these powders.
Particle size in the micrometer range, spherical shapes, crystallinity and purity and lack of voids in particles are essential qualities of the powders for electronic applications such as in multilayer capacitors. Spray decomposition of solution droplets of precursor compounds is an attractive process because of simple apparatus and equipment design and ability to operate in a continuous mode.
The prior art has developed a number of methods for producing small particle metal powders. For example, Nagashima et al. (J. Material Research, 1987) produced nickel powders by spraying nickel nitrate and chloride solutions followed by in-flight treatment at 900-1600.degree.C. in an H.sub.2 /N.sub.2 atmosphere. Temperatures above 1500.degree. C. were necessary to obtain smooth spheres. Stopic et al. (1996)--(I.J. of Powder Metallurgy, Vol. 32, pp. 59-65) used long residence times of the order of 20 seconds to obtain smooth spheres at lower temperature in the range of 800-900.degree. C. At lower temperatures, a significant amount of nickel oxide, an undesirable by-product, was observed. Although complete conversion to nickel at 900.degree. C. was reported, crystallization was not complete. Hollow spheres were also observed.
Asada et al. (JP 62-1807, 1987) describe a method for production of powders of Ag, Pd and their alloys by decomposition of sprays at temperatures well above the melting point of the metals. Asada et al. (JP Kokai 6-172802, November 1992) mention the addition of trace quantities of calcium and similar elements in producing Pd powder resistant to oxidation.
Kodas et al. (U.S. Pat. No. 5,429,657, July 1995) disclose a method for producing powder of Pd, Ag and their alloys at temperatures significantly lower than the melting point of the metal but at long residence times of 9-20 seconds. The aerosol concentrations were limited to a value which might result in a 10% reduction in concentration by coagulation.
Glicksman et al. (U.S. Pat. No. 5,616,165, April 1997; European Application EP 0761349 A1) extended the same method as Kodas et al. (U.S. Pat. No. 5,429,657, July 1995) to producing gold (Au) powder wherein the operating temperature is below the melting point of gold.
An object of the present invention is to provide an improved method for obtaining micrometer size and submicrometer size metallic powders.